Static noise is one of the related parameters generally used in ADCs (analog-to-digital converters). However, static noise is a static parameter and thus is not suitable for use in high speed ADCs. Moreover, ENOB (effective number of bits) is another key parameter for use in ADCs. The state-of-the-art measuring techniques utilize signal generators with high-speed and high-resolution to generate sinusoids that are pure enough. However, commonly used signal generators fail to meet the requirements for both high-speed and high-resolution. In lack of high quality signal generators, there is therefore a need for developing a method for ENOB estimation.
Several specifications must be examined when one desires to choose an ADC. Some of these parameters vary with different input frequencies, while others do not. The parameters that do not vary with different input frequencies are categorized as static parameters. The major static parameters for use in ADCs are differential non-linearity (DNL), integral non-linearity (INL), gain error, offset error, static noise, missing code and monotonic.
Since the parameters mentioned above can not vary with different input frequencies and thus fail to serve as parameters for evaluating the dynamic performance of a high-speed ADC. The major dynamic parameters for use in ADCs are signal-to-noise ratio (SNR), total harmonic distortion (THD), SNR and distortion (SINAD), Spurious-free dynamic range (SFDR), and effective number of bits (ENOB).
Another dynamic parameter for use in ADCs is dynamic deviation that is defined as the number of bits of instability in the output code under the condition that an ADC samples a constant level of a full-scale sinusoid.
Clock jitter is one of the factors that contribute to parameter degradation when an ADC is operated at high-speed and high-resolution. The jitter coming from the aperture uncertainty in the clock source produces enormous side lobes, degrading the dynamic performance. Fast-slewing square-wave clock source is recommended for testing high-speed and high-resolution ADCs. The clean, linear analog supply is used to avoid noises that could degrade ADC parameters.
The digital outputs of ADCs are collected in a high-speed data-capture memory. The collected data are then transferred to a computer for storage and analysis. There are two approaches to ENOB calculation from the collected data. One is derived from a formula by performing FFT (Fast Fourier Transform) operations. A windowing function is necessarily used to avoid errors resulting from spectral leakage due to non-coherence. The other way to calculate ENOB computes RMS error by using a least-mean-square curve fitting technique.
No matter which approach is chosen for ENOB calculation of a high performance ADC, signal generators with high-resolution and high-speed are required to provide sinusoids that are pure enough. If the provided sinusoids are not pure enough, the dynamic performance of an ADC is thus difficult to evaluate.